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Questions About Sap Flow Through Evaporator

I'll be boiling for the first time this year and I'm diving straight into the deep end....! I bought a house that already has a Leader Patriot with a 3X3 syrup pan and a 3X5 raised flue pan, continuous flow. I did a test boil with water this past weekend and think I have a decent handle on how to operate it, but I'm wondering about some of the process flow details. For example:

1) I have 100 CV taps on new gravity lines so I should get some decent sap volume. Based on the size of the evaporator, I'm guessing I need roughly 100gal to fill both pans. Obviously I'll need more to follow it as it starts to boil. As I sweeten the sap, if I don't have enough sap volume to finish syrup, can I leave the sweetened sap in the evaporator? I've seen some people say yes, some drain it. Anybody with a raised flue have experience just leaving the sap in and boiling it again during the next run? Ideally I'd hope that I'd just have some amount of sweetened sap in the evaporator during the whole season, chasing it with fresh sap during each new run. Easier than draining and cleaning each time but I don't know if that's feasible.

2) If I do drain the sweet and keep it cold, can I just add it back in to the next boil?

3) I'm wondering how you finish a boil, say at the end of the season. For example, at some point I won't have any more sap to feed into the evaporator but I'll still want to finish off the sap already in it to syrup. In this case, do I just chase the sweet with water until I've boiled down all the sap? Or do I just assume I'm going to lose some sap at the end of the season?

I wouldn't think 100 taps would feed a 3x8 evaporator for very long, make sure you have #1- a 5 gallon pail of sap for emergencies, and #2 maybe a couple hundred gallons of sap ahead so you can boil for an extended period of time.
Leave the sweetened sap in the pans unless weather is looking like extreme freezing, in that case either drain the pans or light small fires under the pans to take the pressure off the pans from the ice. One reason for draining the pans is if they are lead soldered it is less contact time for the sap to the solder joints.
To finish off the season, all you are really concerned about is what you have in your syrup pan to finish off. Boil it down carefully as low as you can without burning, then after everything is cold finish it off on your stove. If you are really intent on finishing off in your crimp pan also, you can drain it while it is cold, pour it into your syrup pan, and boil water in your crimp pan while you are trying to boil down the sweet in your syrup pan

I think I can help on a few points. 100 taps, estimate 100 gal on a good day. For 3*8, to estimate boil rate, take 3*8=24 - .25 percent if no blower, so around 18 gal per hour with no bells and whistles.

1. You said it will take 100 gal to "fill" both pans. You don't want to fill the pans. For a 3*8 rig, I would guess more like 25-35gal to bring it to the right sap level before you start firing. The right level for just starting out is 1.5 - 2" of depth in the front (syrup) pan, and 1.5-2" above the metal in the back (flue) pan. It is more efficient to run the sap depth at less than 2", but not less than 3/4", ever.

2. In my opinion, do not drain. Unless it is going to be cold for an extended period of time. Reason to drain is it can cause damage to equipment.

3. At the end of the season, when you are out of sap and only have what is in your rig left. Drain flue (back pan) while plugging syrup (front pan). Fill back pan with water, this time fill half way and maintain that level with water. So long as it always covers the metal you will be ok. Then when water is in back, add sap that was in back to front slowly until it is all added in while boiling. Once reduced and last syrup has bee taken off. You can take the rest off and finish on stove top. While making sure to replace with water if heat is still in fire. Never have heat in evaporator without liquid "protecting" pans.

Something I forgot to mention, is that you will need to figure out a way to continuously add sap into the float box on the flue pan from your feed tank in a way to maintain that less than 2" depth across the entire evaporator while continuously applying as much heat in the fire box as possible. This is called continuous flow, and is what will make the syrup come out of the valve in the front (syrup) pan. The sap, a lighter material, will push the "closer to syrup", a heavier material around until the heaviest material, Syrup reaches the valve.

I may have complicated a simple thing. Others may be able too explain better.

OakCorner, while my evaporator is only a 2x6, perhaps my experience can give you some idea of what to expect. Mine requires about 20 gallons to flood, and yours is roughly double the capacity, so expect about 40 gallons to flood. Mine evaporates 30 gph (no air), so expect about 60 gph. Since short boils are inefficient, you will want to boil for at least 3 hours. You would require 220 gallons in your tank to start, plus enough sap to "coast" while your rig cools after the last stoke. Mine requires about 15 gallons, so you might need 30. Learn this amount and remember to stop when this much remains in the tank.

During cold periods between boils, I find that a trouble light placed under the flue pan is warm enough to prevent the flues from freezing.

At the end of the season, I prefer to "chase the sweet" by adding water to the tank when it is empty. I get a couple more draws, then call it quits for the season.

Great info - thanks for the responses! Sounds like I'll need to expand my gravity lines next year to get some more volume (I've only got ~ 1/3rd of the trees available tapped now). Looks like I'll be busy next fall.... Thanks again!

iPakiz you are right. I just re-read what i wrote and realized I forgot a key to the formula.

My understanding is you take your arch's sq / ft, multiply by 3 and -.25% if no blower.

So for this 3*8 it would be 24 * 3 = 72 - .25% (18) for an estimated total boil rate of 54 gph. To gain more efficiency you can add 5-10% for a hood and preheat. Get the 25% back if made airtight and adding a blower.

I agree with the 50-55 gph estimate. You want a minimum of 3 hrs boil before you start your first boil. This will give you enough sugar sweet in the pans to keep it from freezing and damaging the pans, so 165 gal min, and 200+ is even better. Don't boil when the upcoming forecast calls for near zero temperatures. Then have fun. If after sweetening the pans you get a cold snap, either run a small fire each day to liquify the sweet. The sweet will get slushy, but that small fire is enough to protect the pans. Another trick is to just place a 100 watt lite bulb in the firebox and leave it lit. This will give enough heat to protect too. If the temps only fall to the low 20's, you don't need to do anything. The bulb method will allow for lighter syrup than the small fire, because every time you reheat syrup it gets darker. I'm not familiar with the flues on the Patriot raised flue pan, but I "think" my Thor with 10" high flues takes between 45-50 gal to fill to 1" over the raised flues and 1" deep in the syrup pan.
Have fun.

Not to take away from the original post, but I also have a similar question. I have a half pint evaporator chambered pan 2x3. I've only done batch boiling in the past. When I'm all out of sap and have lets say 4 gallons of nearly finished syrup in the pan, can I push that syrup out of the pan with water. And if I do that will I be diluting the syrup in the process ?was hoping to have been boiling already but don't even have my taps in due to the darn weather. Any help would be appreciated.